Abstract

Escherichia coli strains isolated from fecal samples were screened to examine changes in phenotypic and genotypic characteristics including antimicrobial susceptibility, clonal type and carriage of resistance determinants. The goal of this 197 day study was to investigate the influence of administration of chlortetracycline alone (T) or in combination with sulfamethazine (TS) on development of resistance, dissemination of defined strain types and prevalence of resistance determinants in feedlot cattle. Inherent tetracycline resistance was detected from cattle with no prior antimicrobial exposure. Antimicrobial administration was not found to be essential for maintenance of inherent ampicillin resistant (Ampr) and tetracycline resistant (Tetr) E. coli in control animals, however, higher Tetr E. coli shedding was observed from the two treatments. At day 0, high tetracycline (26.7%), lower sulfamethoxazole-tetracycline (19.2%) and several other resistances were detected which by finishing phase (day 197) were restricted to ampicillin-tetracycline (47.5%), tetracycline (31.7%) and ampicillin-tetracycline-sulfamethoxazole (20.8%) from both treated and untreated cattle. Among the determinants, blaTEM1, tet (A) and sul 2 were prevalent at day 0 and 197. Further, E. coli from day 0 showed diverse antibiogram profiles and strain types which by finishing phase were limited to up to 3, irrespective of the treatment. Some genetically identical strains expressed different phenotypes and harboured diverse determinants indicating that mobile genetic elements contribute to resistance dissemination. This was supported by an increased linked inheritance of ampicillin and tetracycline resistance genes and prevalence of specific strains at day 197. Animals in the cohort shed increasingly similar genotypes by finishing phase due to animal to animal strain transmission. Thus characterizing inherent resistance and propagation of cohort-specific strains is crucial for determining antimicrobial resistance in cattle.